e/m ratios for electrons

What do you know about electrons? Well, probably you know that they are little particles that fly around the nucleus of atoms. At least, you would know that if you ever studied atoms in school, which most people are supposed to do. So we have these little particles called electrons and here's something else: they have a charge. An electrical charge. So you can use electrical things to talk about electrons like voltage and current and stuff like that. Also, have you ever played with magnets? And you know how if you put the two south poles close together, they push each other away, but that the north and south poles like each other? Well, the magnets have something called a magnetic field. It's kind of related to electricity, so we can use magnetic fields and electric stuff to talk about electrons.

That's what I did in physics today. Actually, we were finding the ratio between the electron's charge and its mass, e/m.

We used a neat little equation and some very interesting equipment. We had a glass vacuum globe filled with a gas. Then we shot a stream of electrons into the globe and had a magnetic field nearby. The magnetic field caused the electrons to all move in a large circle. The electons had energy which they gave to the gas in the globe causing the gas to glow green. That was how we could see that the electrons were moving in a circle. There was a large green circle in the globe. By playing with some knobs, we adjusted the current which made the electron circle grow bigger or smaller. When we had gotten the circle to the right size, we looked to see how much current we were using. Then we could use this nice little equation:

e/m = 2V/r²B²

The V means the voltage we used, the r is the radius of the circle of electrons, and the B is the magnetic field. In our experiment, B was 7.80*10 to the -4th power times I. I is the current we found.

It was a great deal of fun.